1. Field Of The Invention
The present invention relates to raster scanned video display devices, and more particularly, to a method and apparatus for selectively blending or comparing two or more digital images on a pixel by pixel basis and displaying the results on a display monitor.
2 Description Of The Prior Art
Typically, image data for display on a raster scanned display device is stored in a circuit called a frame buffer. The frame buffer is preferably wide enough to accommodate the largest image display mode. Typical frame buffer widths are 8, 16 or 24 planes, where a plane is normally one bit of information wide. The raster images are drawn by processing the data value stored in the frame buffer to arrive at values suitable for display.
For example, as shown in FIG. 1, an image frame buffer 102 having 24 planes of information and three bits of window/overlay plane information may provide its outputs to display mode controller 104, where a display mode, such as full color, eight-bit indexed, and the like is determined. The data output of the display mode controller 104 is typically post-processed in another device called a color map, also known as a color table, color look-up table or color ROM. As shown, a different color map may be provided for the red (106), green (108) and blue (110) colors. The outputs from the display mode controller 104 are typically provided as indices to the respective color maps, and for the eight plane system shown, 2.sup.8 (=256) intensity values for red and for green and for blue may be respectively provided. Thus, the color intensity values stored in the color maps are accessed for one or more adjacent pixels by using the inputted planes as indices to these intensity values and then outputting the result.
Each color map thus translates the image data into component colors, which are normally, but not necessarily, red, green and blue (RGB). Often data from the image frame buffer 102 is also multiplexed into the color maps to create different display modes by display mode controller 104. Most display devices then use the output of the color maps to feed the input of three digital to analog converters 112, 114 and 116, respectively, to obtain an RGB analog video signal which is then used to drive the video display unit. Of course, for a monochrome display, only one digital to analog converter is necessary.
In such raster scanned frame buffers, it has often been desirable to segment the frame buffer 102 into regions of interest called "windows". Separate and independent windows have been used to provide independent modes of display. Normally, the window regions within the image frame buffer 102 are identified by extending the frame buffer width another three or four planes as shown in FIG. 1 to allow for a tag field, by which each display region may be identified. A tag field of three planes allows for eight unique windows, Where a tag field of four planes allows for sixteen unique window regions.
When it has been desired to combine a plurality of images output from display circuits of the type shown in FIG. 1, the analog RGB outputs of each circuit are added together before being input to the video display unit. In particular, analog RGB outputs for each image to be displayed are summed in an analog adder and then clamped at a predetermined level so as not to oversaturate the video display device. This combining occurs during scanning in order to achieve the proper result.
However, the prior art technique of combining the analog RGB signals for display in this manner has numerous drawbacks. In particular, since the blending occurs after the digital to analog conversion, the blending is difficult to achieve on a pixel by pixel basis, and accordingly, window boundaries may not be properly distinguished for display. Moreover, only one color map is provided for the entire display device, and thus separate color maps for each image may not be provided. In addition, the prior art technique has problems in that the digital to analog converters must be matched to each other or else intensity changes will occur between respective images. Blending of the signals in this manner also causes bandwidth problems, for since processing is performed in analog the summing amplifier must have extremely high bandwidths.
Similarly, when it has been desired to compare images output from display circuits of the type shown in FIG. 1, the analog RGB outputs of each circuit are subtracted from each other before being input into the video display unit. In particular, analog RGB outputs for each image to be compared are subtracted from each other in an analog subtractor and the difference is displayed during scanning. However, as with image blending, such comparison is difficult to achieve on a pixel by pixel basis.
Accordingly, there is a need in the art for a process and apparatus which enables images to be selectively blended or compared on a pixel by pixel basis so that the result can be displayed. In addition, there is a need in the art for a process and apparatus which compares input images on a per pixel basis to determine the differences therebetween and which can overlay this image on one of the original images to show where in that original image it differs from the other image. The present invention has been designed for these purposes.